Difficulty
Easy
Sudoku technique guide
Box-Line Reduction
When the easy moves slow down, one clear pattern can make the next step feel possible again. Box-Line Reduction helps you spot what the grid is already telling you.
By the end, you will know what to notice, how the idea works and when it is worth using.
At a glance
Technique family
Locked candidates
Purpose
Removes candidates
Typical puzzle stage
Early to middle
Frequency
Common
Prerequisites
Candidate marks and line scanning
Why this technique matters
Box-Line Reduction helps you see a row or column as a source of information for a box. It is one of the most useful early candidate eliminations.
The idea in plain English
When all possible positions for one digit in a row or column fall inside one 3x3 box, that digit cannot appear anywhere else in that box.
How to recognise it
A candidate appears only inside one box when you scan across a whole row or down a whole column.
Pause when you see this shape. You do not have to force it; just check whether the pattern is complete.
How the logic unfolds
Think of this as a short conversation with the grid. Take one check at a time, and let each answer tell you what to inspect next.
Start by. Pick a missing digit in one row or column.
Then. Check all cells in that line where the digit could go.
Next. If every possible position lies in the same box, the line locks that digit into the box.
Finally. Remove the digit from the other empty cells in that box.
When it usually appears
Use it after candidate marks become crowded in a row or column. It often appears alongside Pointing Pairs.
Common traps
These are easy places to get turned around. They are useful checks, not signs that you are doing badly.
- Looking only at the box. This pattern starts with the whole row or column.
- Removing the candidate from the locked positions instead of from the other cells in the box.
Frequently asked questions
Is this the same as a Pointing Pair?
It is the reverse direction. Pointing starts in a box; Box-Line Reduction starts in a row or column.
Can it place a number immediately?
Usually it removes candidates first, but that removal may reveal a single.
Quick takeaway
- A Box-Line Reduction belongs to the locked candidates family.
- Its purpose is to remove candidates.
- Look for a complete pattern, not one that only nearly fits.
- It is most useful early to middle.
- Once you make the deduction, return to the simpler scans.